Assuming you have already created a DNS zone for your domain following these instructions: https://billing.dotblock.com/index.php?rp=/knowledgebase/168/How-do-I-add-a-DNS-zone-for-my-domain.html
To add an SRV record to your domains DNS zone, to the right of the + sign to add a record is a dropdown menu, which has an option for Add SRV record. Clicking that will open the SRV record wizard which makes it a bit easier to add SRV records:
The example above is a common SRV record for Microsoft Office 365 which accomplishes this:
Service Protocol Port Weight Priority TTL Name Target
_sip _tls 443 1 100 3600 @ sipdir.online.lync.com
Note: Keep in mind that it will automatically add the underscore to the record, so if you are trying to create an srv record for _sip you would only enter in sip in the service field